Identification and characterization of meiotic drive within the Drosophila virilis subgroup

An organism’s DNA is organized into chromosomes, which are structured to help ensure proper transmission from parent to offspring. Major alterations to these chromosomes can lead to many negative fitness effects, yet there is a vast diversity in chromosome size, shape, and number among, and in some cases within, species. The mechanisms that drive such chromosome diversity are poorly understood. I investigated the role of biased segregation in female meiosis, i.e., meiotic drive, as a force that contributes to the evolution of chromosome form. More specifically, I investigated how biased segregation in females can facilitate the evolution of two acrocentric chromosomes (centromere at one end of the chromosome) fusing together to make one large metacentric (centromere at the center of the chromosome) chromosome. The closely related species pair, Drosophila americana and Drosophila novamexicana, are exemplars for understanding mechanisms of chromosomal evolution. Since their recent divergence (~half million years), D. americana has evolved two different centromeric fusions: one fusion between the 2nd and 3rd chromosomes, and the other fusion between the X and 4th chromosomes. The 2-3 fusion is fixed in D. americana; however, the X-4 centromeric fusion remains polymorphic within the species. We show that heterozygous females, with respect to the X-4 fusion and the 2-3 fusion, show significantly biased transmissions for both the X-4 fusion (57%) and the 2-3 fusion (62%). Yet, in D. novamexicana, there is no meiotic drive for either the fused X-4 or the unfused X and 4th chromosomes. I have also begun to uncover the role of the centromeric regions in the transmission bias. The centromere itself is not sufficient to induce meiotic drive but rather an interaction with a secondary factor which may allow the fused centromeres to exploit meiosis to increase inheritance from mother to offspring. I also measured heterochromatin content between the fused and unfused X and 4th homologs. No obvious size differences were uncovered, but possible compositional differences were revealed. This suggests that if the centromere itself is involved in meiotic drive, the difference in number of centromeres or compositional differences between the centromeres may be factors in meiotic drive. Overall, I have identified and characterized meiotic drive as a force driving karyotype evolution in D. americana, and I have begun dissecting the mechanisms of meiotic drive.